In many communication systems, a local oscillator is used to demodulate an input signal to remove a carrier signal. In such conventional communication systems, a feedback loop, such as a Costas loop, may be used to control the frequency and the phase of the local oscillator. The feedback loop synchronizes the local oscillator and the carrier signal, such that the signal generated by the local oscillator may be mixed with the input signal to suppress the carrier signal leaving only the information portion of the signal. Most phase lock loops, such as the Costas loop, require a predetermined time period in which to latch or lock onto the carrier frequency, so that the local oscillator is controlled to suppress the carrier signal within a predetermined performance criteria.
High speed modems utilized in time division multiple access systems require rapid acquisition of the carrier frequency and phase, in addition to rapid acquisition of the bit timing and the data timing. In conventional communication systems, these four required pieces of information are often derived in a sequential fashion. This sequential operation requires the use of an excessive percentage of the allotted signal burst length in a multiple access system.
In a conventional coherent, carrier demodulation phase lock loop operating with bi-phase shift keyed modulation, the incoming signal is an intermediate frequency (IF) derived by down converting the incoming radio frequency signals to a standard frequency of, for example, seventy megahertz. This signal is mixed with the output from a Voltage Control Oscillator (VCO) or its digital counterpart, a Number Controlled Oscillator (NCO). The VCO or NCO output is at the center frequency of the incoming IF signal and is utilized to mix the incoming signal down to base band in two mixers known as the in-phase mixer and the quadrature phase mixer. The in-phase mixer is driven from the VCO (or NCO) through a 90.degree. phase shifter. When carrier lock is achieved, the VCO (or NCO) output and the IF input will be in-phase so that the VCO (or NCO) signal received at the in-phase channel mixer is in phase with the carrier signal. The output of the in-phase mixer is low pass filtered and becomes the data output.
The quadrature mixture operates in phase quadrature with incoming signals to produce an output at base band that has nominal zero DC value and is utilized to tune the VCO (or NCO) in a null seeking loop. In a standard Costas loop the in-phase channel output is limited and multiplied with the quadrature channel output to produce a loop control voltage.
To determine bit timing and data timing, it is necessary to detect a code word in the input signal which is used as a preamble to the time division multiple access signal burst. In the prior art, a transversal correlator performed a matched filter detection of the code word in both the in-phase and quadrature phase channels. The correlator outputs are normally full wave rectified and summed to provide a trigger pulse to start the bit and word timing circuits. In such prior art systems, there existed a statistical probability of failure of the Costas carrier recovery loop to lock on the carrier signal and settle in the required period of time after the code signal was detected to initiate data recovery. Thus, a need has arisen for a communication system in which the recovery loop rapidly settles to lock the local oscillator in synchronism with the carrier signal.